Optical transmission systems, which use coherent (laser) light and optical fibers to send data, often employ phase modulation to encode data in an optical carrier wave.
The phase-modulated light must eventually be received and demodulated by an optical receiver to yield electrical signals. It is vital that the receiver be capable of receiving, demodulating and decoding the data contained in the optical signal as quickly as it is received.
Some traditional phase demodulation techniques rely on a phase discriminator (typically a simple delay-line filter), an envelope detector and an integrator. Unfortunately, phase discriminators exhibit nonlinearities, which limit their dynamic range. Other phase demodulation techniques rely on fast phase tracking. Unfortunately, phase tracking works well for rather small signal bandwidths and low carrier frequencies.
The wide-band rectifier narrowband amplifier (WIRNA) receiver architecture, which originally was developed for amplitude modulation (AM) - coherent systems, has been successfully demonstrated in a coherent phase-modulated link. WIRNA uses a nonlinear device to mix beat tone and a sideband to a lower frequency to cancel out laser phase noise. Unfortunately, this non-linear process exhibits intermodulation distortion (IMD) at high carrier frequencies. Furthermore, the nonlinear device has to operate at two to three times the operating frequency of the link. Therefore, the WIRNA architecture cannot be used for a receiver required to operate with respect to a wide range of center carrier frequencies.
All of the above-described techniques require bandwidths that are several multiples of the incoming signal bandwidth to recover data reliably. Unfortunately, the bandwidth that is required of the receiver becomes immense. Traditional phase demodulation techniques become essentially unworkable.
Accordingly, it may be desirable to have a fundamentally new optical receiver topology that does not require a bandwidth that is several times that of the incoming optical signal. It may also be desirable to have a better way to receive and demodulate optical signals.